<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <style>
        .p{
            position: relative;
            width:600px;
            height:600px;
            border:1px solid red;
        }
        .c{
            width:100px;
            height:100px;
            background:red;
            position:absolute;
            margin: auto;
        }
    </style>
    <div class="p">
       <div class="c"></div>
    </div>
</body>
</html>